Historically, today's most favorite rechargeable energy storage devices—lithium-ion batteries—actually evolved from rechargeable “lithium metal batteries” that use lithium (Li) metal as the anode and a Li intercalation compound (e.g. MoS2) as the cathode. Li metal is an ideal anode material due to its light weight (the lightest metal), high electronegativity (−3.04 V vs. the standard hydrogen electrode), and high theoretical capacity (3,860 mAh/g). Based on these outstanding properties, lithium metal batteries were proposed 40 years ago as an ideal system for high energy-density applications.
Due to some safety concerns of pure lithium metal, graphite was implemented as an anode active material in place of the lithium metal to produce the current lithium-ion batteries. The past two decades have witnessed a continuous improvement in Li-ion batteries in terms of energy density, rate capability, and safety. However, the use of graphite-based anodes in Li-ion batteries has several significant drawbacks: low specific capacity (theoretical capacity of 372 mAh/g as opposed to 3,860 mAh/g for Li metal), long Li intercalation time (e.g. low solid-state diffusion coefficients of Li in and out of graphite and inorganic oxide particles) requiring long recharge times (e.g. 7 hours for electric vehicle batteries), inability to deliver high pulse power (power density<<1 kW/kg), and necessity to use pre-lithiated cathodes (e.g. lithium cobalt oxide, as opposed to cobalt oxide), thereby limiting the choice of available cathode materials. Further, these commonly used cathode active materials have a relatively low specific capacity (typically <220 mAh/g). These factors have contributed to the two major shortcomings of today's Li-ion batteries—a low energy density (typically 150-220 Wh/kgcell) and low power density (typically <0.5 kW/kg).
Although several high-capacity anode active materials have been found (e.g., Si with a theoretical capacity of 4,200 mAh/g), there has been no corresponding high-capacity cathode material available. To sum it up, battery scientists have been frustrated with the low energy density of lithium-ion cells for over three decades! Current cathode active materials commonly used in Li-ion batteries have the following serious drawbacks:                (1) The practical capacity achievable with current cathode materials (e.g. lithium iron phosphate and lithium transition metal oxides) has been limited to the range of 150-250 mAh/g and, in most cases, less than 200 mAh/g.        (2) The production of these cathode active materials normally has to go through a high-temperature sintering procedure for a long duration of time, a tedious, energy-intensive, and difficult-to-control process.        (3) The insertion and extraction of lithium in and out of these commonly used cathodes rely upon extremely slow solid-state diffusion of Li in solid particles having very low diffusion coefficients (typically 10−8 to 10−14 cm2/s), leading to a very low power density (another long-standing problem of today's lithium-ion batteries).        (4) Current cathode active materials are electrically and thermally insulating, not capable of effectively and efficiently transporting electrons and heat. The low electrical conductivity means high internal resistance and the necessity to add a large amount of conductive additives, effectively reducing the proportion of electrochemically active material in the cathode that already has a low capacity. The low thermal conductivity also implies a higher tendency to undergo thermal runaway, a major safety issue in lithium battery industry.        (5) The most commonly used cathodes, including lithium transition metal oxides, contain a high oxygen content that could assist in accelerating the thermal runaway and provide oxygen for electrolyte oxidation, increasing the danger of explosion or fire hazard. This is a serious problem that has hampered the widespread implementation of electric vehicles.        
Thus, there is a strong and urgent need to develop high-capacity cathode active materials. Metal fluorides with metallic cations in high oxidation states and a strong ionic character of the M-F bonds (M=a metal) have been proposed as alternative cathode active materials due to their high theoretical energy densities. For instance, FeF3 has attracted considerable interests because of its low cost and low toxicity. However, the highly ionic character induces a large band gap, thus metal fluorides have very poor electronic conductivity. In addition, LiF, the product of the conversion reaction, is also highly insulating. Accordingly, metal fluoride electrodes often suffer severely from slow reaction kinetics and low lithium storage capacity, significantly lower than the theoretical capacity.
Several attempts have been made to overcome these issues, but with very limited success. For instance, an effort was made to enhance the electrochemical activity by reducing the metal fluoride particle size to the nanometer range for the purpose of achieving shorter electron-conducting paths and larger reaction surface. In this example, Badway, et al. reported a FeF3/C nanocomposite through ball-milling [F. Badway, et al., “Carbon metal fluoride nanocomposites high-capacity reversible metal fluoride conversion materials as rechargeable positive electrodes for Li batteries,” J. Electrochem. Soc. 150 (2003) A1318-A1327]. However, this method has several drawbacks, including difficulties in controlling materials properties and production of a significant number of defects.
The deposition of FeF3 on conductive carbon particle surfaces without ball milling was proposed as another means of improving electrode performance. For instance, Kim et al. have fabricated carbon nanotube/FeF3 composites by nucleation of FeF3 on the defects of CNT surfaces generated by HF etching [S. W. Kim, et al., “Fabrication of FeF3 nanoflowers on CNT branches and their application to high power lithium rechargeable batteries,” Adv. Mater. 22 (2010) 5260-5264]. This strategy has been followed by others to fabricate FeF3 on activated carbon micro bead [L. Liu, et al., “Synthesis and electrochemical performance of spherical FeF3/ACMB composite as cathode material for lithium-ion batteries,” J. Mater. Sci. 47 (2012) 1819-1824]. Liu et al. proposed a low-temperature in situ approach for the synthesis of uniform FeF3 nano particles on reduced graphene oxide (rGO) sheets suspended in ethanol solution [J. Liu, et al., “Mild and cost-effective synthesis of iron fluoride-graphene nanocomposites for high-rate Li-ion battery cathodes,” J. Mater. Chem. A 1 (2013) 1969-1975]. However, the loading level of FeF3 on rGO and the rate capability of the FeF3/graphene composites remain too low for practical applications.
Other attempts to use graphene as a conductive additive for FeF3 all fall short in providing good rate capability, high energy density, and long cycle life. Examples of these earlier efforts are [X. Zhao, et al., “Photothermal-assisted fabrication of iron fluoride-graphene composite paper cathodes for high-energy lithium-ion batteries,” Chem. Commun. 48 (2012) 9909-9911] and [Q. Chu, et al. “Reduced graphene oxide decorated with FeF3 nanoparticles:
Facile synthesis and application as a high capacity cathode material for rechargeable lithium batteries,” Electrochim. Acta. 111 (2013) 80]. Although Q. Chu, et al. claim to achieve a high specific capacity of 476 mAh/g, this capacity is achieved only when the current density is at a practically useless value of 50 mA/g (an excessively low discharge rate). Further, the specific capacity rapidly drops to approximately 110 mAh/g after only 50 charge-discharge cycles (see FIG. 5B of Chu, et al.). Furthermore, this maximum achievable value of 476 mAh/g is significantly lower than the theoretical specific capacity of 712 mAh/g for FeF3, indicating a low active material utilization rate (i.e. a significant proportion of the active material is not fully utilized).
Due to extremely poor electrical conductivity of all cathode active materials in a lithium-ion or lithium metal cell, a conductive additive (e.g. carbon black, fine graphite particles, expanded graphite particles, or their combinations), typically in the amount of 5%-15%, must be added into the electrode. However, the conductive additive is not an electrode active material. The use of a non-active material means that the relative proportion of an electrode active material is reduced or diluted. For instance, the incorporation of 5% by weight of PVDF as a binder and 5% of carbon black as a conductive additive in a cathode would mean that the maximum amount of the cathode active material (e.g., lithium cobalt oxide) is only 90%, effectively reducing the total lithium ion storage capacity. Since the specific capacities of the more commonly used cathode active materials are already very low (140-220 mAh/g), this problem is further aggravated if a significant amount of non-active materials is used to dilute the concentration of the active material.
Carbon black (CB) materials, as a conductive additive, have several drawbacks: (1) CBs are typically available in the form of aggregates of multiple primary particles that are typically spherical in shape. Due to this geometric feature (largest dimension-to-smallest dimension ratio or aspect ratio ˜1) and the notion that CBs are a minority phase dispersed as discrete particles in an electrically insulating matrix (e.g. lithium cobalt oxide and lithium iron phosphate), a large amount of CBs is required to reach a percolation threshold where the CB particles are combined to form a 3-D network of electron-conducting paths. (2) CBs themselves have a relatively low electrical conductivity and, hence, the resulting electrode remains to be of relatively low conductivity even when the percolation threshold is reached. A relatively high proportion of CBs (far beyond the percolation threshold) must be incorporated in the cathode to make the resulting composite electrode reasonably conducting.
Clearly, an urgent need exists for an effective supporting material for metal fluorides and chlorides that enables a high cathode active material utilization rate, high specific capacity at both high and low charge/discharge rates (not just at a low rate), high rate capability, long cycle-life, and improved heat dissipation generated during a battery operation. These are the main objectives of the instant invention.
This supporting or “enabling” material also must be electrically conductive. Preferably, this electrically conductive supporting material is also of high thermal conductivity. Such a thermally conductive additive would be capable of dissipating the heat generated from the electrochemical operation of the Li-ion battery, thereby increasing the reliability of the battery and decreasing the likelihood that the battery will suffer from thermal runaway and rupture. With a high electrical conductivity, there would be no need to add a high proportion of conductive additives.
It is an object of the present invention to provide a cathode layer that exhibits a combination of exceptional thermal conductivity, electrical conductivity, mechanical strength, and elastic modulus unmatched by any cathode layer commonly used in a lithium-ion battery or lithium metal battery.